Hi @Buttbutt, welcome to our community!
Is it the voice mail from the operator that you want to turn off? If that is the case, you will need to get in touch with them and they will assist you further with disabling this feature.
You can also check if you have enabled the Xperia answer machine in your device, but do note that it might not be available due to it depending on the operator specifications. The Xperia answering machine can be found in Settings > Call > Xperia answering machine > turn off "Answering machine".
I hope that answers your question! 
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